Melanins are the major pigments of hair, skin and eyes. Naturally occurring melanins can be of a variety of colours, such as black, brown and red. Two major categories of melanins are known; eumelanins which provide a black or brown pigmentation, and pheomelanins, which provide a yellow to reddish-brown pigmentation. Natural suntanning occurs through exposure of the skin to light radiation at a wavelength of 280-400 nm.
For most individuals the sensitivity of the skin to ultra-violet radiation (hereinafter referred to as "UVR") is largely determined by its melanin content. The amounts and types of melanin present in the skin vary from person to person and, thus, the tolerance of the skin when exposed to UVR varies from person to person. For most individuals the greater the melanin content of the skin of an individual, the more tolerant is the skin of that individual to UVR.
Melanins are complex heteropolymers, of which the biosynthetic pathways and exact chemical structure are unknown. However, melanins are believed to be copolymers of 3,4-dihydroxyphenylalanine (DOPA) and cysteinyldopa. While the inventors do not intend to be limited to any particular scheme of synthesis, it is believed that the in vivo synthetic pathway leading to the production of melanin begins with the enzymatic oxidation of the amino acid, tyrosine, to form DOPA, and continues in the presence of the enzyme tyrosinase (polyphenol oxidase) with a series of oxidation and polymerization reactions to result in melanin. This postulated synthetic pathway may be represented as follows: ##STR1##
Melanins may also be produced by the auto-oxidation of DOPA, whereby the production of melanin occurs in the absence of tyrosinase.
Thus, it is believed that known melanins, or compounds with the same or similar functional features as known melanins, may be produced either naturally or synthetically from DOPA, analogs of DOPA or biosynthetic precursors of DOPA. As used herein, "melanins" includes all such known melanins or melanin products, however derived, and all melanin-like compounds or analogs with the herein-described pigment and photoprotective functions.
The photoprotective function of melanin is believed to be associated with both its physical and biochemical properties. Melanin is believed to dissipate UVR into heat which is absorbed, and to promote oxidation reactions and quench free radicals, the formation of which is induced by UVR. Melanin is able to function as a stable free radical, thus resulting in its ability to quench damaging free radicals formed in the skin on exposure to UVR. The role of melanin as a free radical scavenger is significant in its photoprotective role in individuals having a moderate or greater melanin content in their skin; however, it is highly desirable that individuals having a minimal melanin content in their skin supplement the body's natural protective mechanism to UVR with a topical sunscreen supplement. It is now recognized that even individuals with moderate or greater levels of melanin should supplement this natural protective mechanism when longer exposure to UVR is anticipated.
As a result of the increasing awareness of the public of the detrimental effects of exposure to UVR, a variety of sunscreen compositions for topical administration have been developed. Various salicylate compounds have been described as sunscreen agents due to their ability to absorb UVR. For example, U.S. Pat. Nos. 3,506,758 and 4,256,664 describe compositions for use as sunscreens which include the esterification product of p-aminobenzoic acid (paba) and/or salicylic acid with lecithins, choline and/or imidazoles having a reactive hydroxyl group. U.S. Pat. No. 4,454,112 describes tocopherol acetylsalicylate compounds which are useful as sunscreen agents in compositions for topical administration. However, it has been found that salicylate compounds may cause skin irritations and allergic reactions in some individuals, and, as a result, are not ideal as the primary components of such topical compositions.
A common UVR absorbing compound found in sunscreen compositions is paba, and esters of paba, such as octyl dimethyl paba [octyl-p-(dimethyl-amino) benzoate]. U.S. Pat. No. 4,434,154 teaches one such UVR protective composition which contains an emulsion of octyl dimethyl paba, dihydroxy acetone, water, oil and a surfactant.
Sun protectant compositions have also been described which contain melanin. However, the known methods of making melanin have disadvantages. With both the enzymatic and auto-oxidation methods for preparing melanins, the properties of the melanin product obtained make it undesirable for incorporation into a sunscreen composition for application onto the skin. Specifically, the melanins obtained are granular in nature, which results in a product that cannot be spread evenly when applied to the skin. These melanins are also undesirable in that they are not easily absorbed by the skin under acceptable physiological skin conditions (i.e. neutral pH). Further, the effectiveness of these melanins is limited with respect to ultra-violet absorption. Specifically, the absorption capacity of the melanins produced by either of these methods is low in the short-wave range (UVB) of the ultra-violet spectrum. U.S. Pat. No. 4,806,344 describes one such melanin composition additionally comprising ferric chloride, triethanolamine, a sunscreen compound such as paba and a cosmetic base in the form of an oil, cream or ointment.
Many of the sunscreen products currently available do not fully protect the skin against the complete wavelength spectrum of UVR. UVR comprises both long-wave UVR (or UVA, having a wavelength of approximately 320-400 nm) and short-wave UVR (or UVB, having a wavelength of approximately 290-320 nm). Of particular concern for a large portion of the population is protection of the skin against the aging process which is accelerated by exposure to UVR. Long-wave UVA is believed to specifically contribute to the aging process.
Most ultra-violet light absorbing compounds useful as sunscreen agents only absorb within a small range of the wavelength of UVR. Thus, in order to maximize the absorption capacity of UVR in a sunscreen composition, the composition may contain not just a single sunscreen compound but several such compounds which are selected to provide broad range UVR protection. However, sunscreen compositions containing a variety of compounds are more apt to cause allergic or photoallergic reactions in individuals. Allergic and photoallergic reactions to existing sunscreen compounds and compositions is a common occurrence and generally results in dermatitis or photodermatitis. Certain individuals also suffer from skin eruptions due to long UVA and visible light, and a very broad wavelength sunscreen would be valuable for these people.
Thus, it would be desirable to provide a sunscreen composition which imparts effective protection against the broad spectrum of UVR, which is compatible for use with human skin tissue.